Method and apparatus for varying an impedance

ABSTRACT

A method and apparatus for dynamically varying the impedance of a tank circuit whereby, over time, the response of the circuit to a received signal is maximized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to variable impedances, and, inparticular, to a variable impedance for use in a tank circuit.

2. Description of the Related Art

In general, in the descriptions that follow, I will italicize the firstoccurrence of each special term of art which should be familiar to thoseskilled in the art of radio frequency (“RF”) communication systems. Inaddition, when I first introduce a term that I believe to be new or thatI will use in a context that I believe to be new, I will bold the termand provide the definition that I intend to apply to that term. Inaddition, throughout this description, I will sometimes use the termsassert and negate when referring to the rendering of a signal, signalflag, status bit, or similar apparatus into its logically true orlogically false state, respectively, and the term toggle to indicate thelogical inversion of a signal from one logical state to the other.Alternatively, I may refer to the mutually exclusive boolean states aslogic_(—)0 and logic_(—)1. Of course, as is well know, consistent systemoperation can be obtained by reversing the logic sense of all suchsignals, such that signals described herein as logically true becomelogically false and vice versa. Furthermore, it is of no relevance insuch systems which specific voltage levels are selected to representeach of the logic states.

In general, in an RF communication system, an antenna structure is usedto receive signals, the carrier frequencies (“f_(C)”) of which may varysignificantly from the natural resonant frequency (“f_(R)”) of theantenna. It is well known that mismatch between f_(C) and f_(R) resultsin loss of transmitted power. In some applications, this may not be ofparticular concern, but, in others, such as in RF identification(“RFID”) applications, such losses are of critical concern. For example,in a passive RFID tag, a significant portion of received power is usedto develop all of the operating power required by the tag's electricalcircuits. In such an application, it is known to employ a variableimpedance circuit to shift the f_(R) of the tag's receiver so as tobetter match the f_(C) of the transmitter of the system's RFID reader.

Although it would be highly desirable to have a single design that isuseful in all systems, one very significant issue in this regard is thelack of international standards as to appropriate RFID systemfrequencies, and, to the extent there is any de facto standardization,the available frequency spectrum is quite broad: Low-Frequency (“LF”),including 125-134.2 kHz and 140-148.f kHz; High-Frequency (“HF”) at13.56 MHz; and Ultra-High-Frequency (“UHF”) at 868-928 MHz. Compoundingthis problem is the fact that system manufacturers cannot agree on whichspecific f_(C) is the best for specific uses, and, indeed, to preventcross-talk, it is desirable to allow each system to distinguish itselffrom nearby systems by selecting different f_(C) within a defined range.

As explained in, for example, U.S. Pat. No. 7,055,754 (incorporatedherein by reference), attempts have been made to improve the ability ofthe tag's antenna to compensate for system variables, such as thematerials used to manufacture the tag. However, such structuralimprovements, while valuable, do not solve the basic need for a variableimpedance circuit having a relatively broad tuning range.

Shown in FIG. 1 is an ideal variable impedance circuit 2 comprised of avariable inductor 4 and a variable capacitor 6 coupled in parallel withrespect to nodes 8 and 10. In such a system, the undamped resonance orresonant frequency of circuit 2 is:

$\begin{matrix}{\omega_{R} = \frac{1}{\sqrt{LC}}} & \left\lbrack {{Eq}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

-   -   where: ω_(R)=the resonant frequency in radians per second;        -   L=the inductance of inductor 2, measured in henries; and        -   C=the capacitance of capacitor 6, measured in farads.

On, in the alternative form:

$\begin{matrix}{f_{R} = {\frac{\omega_{R}}{2\;\pi} = \frac{1}{2\;\pi\sqrt{LC}}}} & \left\lbrack {{Eq}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

where: f_(R)=the resonant frequency in hertz.

As is well known, the total impedance of circuit 2 is:

$\begin{matrix}{Z = \frac{RLS}{{RLCS}^{2} + {LS} + R}} & \left\lbrack {{Eq}.\mspace{14mu} 3} \right\rbrack\end{matrix}$

-   -   where: Z=the total impedance of circuit 2, measured in ohms;        -   R=the total resistance of circuit 2, including any parasitic            resistance(s), measured in ohms;        -   L=the inductance of inductor 2, measured in henries; and        -   S=jω;        -   where: j=the imaginary unit √{square root over (−1)}; and            -   ω is the resonant frequency in radians-per-second.

As is known, for each of the elements of circuit 2, the relationshipbetween impedance, resistance and reactance is:Z _(e) =R _(e) +jX _(e)   [Eq. 4]

-   -   where: Z_(e)=impedance of the element, measured in ohms;        -   R_(e)=resistance of the element, measured in ohms;        -   j=the imaginary unit √{square root over (−1)}; and        -   X_(e)=reactance of the element, measured in ohms.

Although in some situations phase shift may be relevant, in general, itis sufficient to consider just the magnitude of the impedance:|Z|=√{square root over (R ² +X ²)}  [Eq. 5]

For a purely inductive or capacitive element, the magnitude of theimpedance simplifies to just the respective reactances. Thus, forinductor 4, the magnitude of the reactance can be expressed as:X _(L) =|j2πfL|=j2πfL  [Eq. 6]

Similarly, for capacitor 6, the magnitude of the reactance can beexpressed as:

$\begin{matrix}{X_{C} = {{\frac{1}{j\; 2\;\pi\;{fC}}} = \frac{1}{2\;\pi\;{fC}}}} & \left\lbrack {{Eq}.\mspace{14mu} 7} \right\rbrack\end{matrix}$

Because the reactance of inductor 4 is in phase while the reactance ofcapacitor 6 is in quadrature, the reactance of inductor 4 is positivewhile the reactance of capacitor 6 is negative. Resonance occurs whenthe absolute values of the reactances of inductor 4 and capacitor 6 areequal, at which point the reactive impedance of circuit 2 becomes zero,leaving only a resistive load.

As is known, the response of circuit 2 to a received signal can beexpressed as a transfer function of the form:

$\begin{matrix}{{H\left( {j\;\omega} \right)} = \frac{\frac{1}{R} + {j\left( {{{- C}\;\omega} + \frac{1}{L\;\omega}} \right)}}{\frac{1}{R^{2}} + \left( {{{- C}\;\omega} + \frac{1}{L\;\omega}} \right)^{2}}} & \left\lbrack {{Eq}.\mspace{14mu} 8} \right\rbrack\end{matrix}$

Within known limits, changes can be made in the relative values ofinductor 4 and capacitor 6 to converge the resonant frequency, f_(R), ofcircuit 2 to the carrier frequency, f_(C), of a received signal. As aresult of each such change, the response of circuit 2 will get stronger.In contrast, each change that results in divergence will weaken theresponse of circuit 2.

A discussion of these and related issues can be found in the MastersThesis of T. A. Scharfeld, entitled “An Analysis of the FundamentalConstraints on Low Cost Passive Radio-Frequency Identification SystemDesign”, Massachusetts Institute of Technology (August 2001), a copy ofwhich is submitted herewith and incorporated herein in its entirety byreference.

I submit that what is needed is an efficient method and apparatus fordynamically varying the impedance of a tank circuit, and, in particular,wherein the impedance of the circuit can be efficiently varied so as todynamically shift the f_(R) of the circuit to better match the f_(C) ofa received signal and thereby improve the response of the circuit.

BRIEF SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of my invention, I provide amethod for dynamically varying the impedance of a tank circuitcomprising an inductor and a capacitor, the capacitance of which can beselectively varied. According to my method, I first select whichdirection I will initially vary the capacitance, that is, either up ordown. I then capture the current response of the circuit to a receivedsignal. Next, I vary the capacitor in the direction I just selected.Then, I compare the captured response to the current response, and, ifthe comparison indicates that the current response is weaker than thecaptured response, I change my selection for which direction that I willnext vary the capacitance, and repeat the capture, vary and comparesteps. Although my method is inherently recursive in nature, it will beappreciated that the initially selected rate of recursion can beincreased, decreased or even stopped under appropriate conditions.

In accordance with another preferred embodiment of my invention, Iprovide a tuning circuit for dynamically varying the impedance of a tankcircuit comprising an inductor and a capacitor, the capacitance of whichcan be selectively varied. In my preferred tuning circuit, a referencevoltage generator, coupled to the tank circuit, produces a referencevoltage proportional to the response of the tank circuit to a receivedsignal. A differentiator, coupled to the generator, then determines thepolarity of the change in the reference voltage between a first point intime and a second point in time. Next, a direction selector, coupled tothe differentiator and adapted to respond to the determined polarity,selects one of two different directions in which the capacitance of thecapacitor can be varied. Finally, a ramp generator, coupled to theselector and to the capacitor, selectively varies the capacitance of thecapacitor in the currently-selected direction. As with my preferredmethod, my preferred tuning circuit is inherently recursive inoperation, and the initially selected rate of recursion can be easilyincreased, decreased or even stopped under appropriate conditions.

I submit that each of these embodiments of my invention more efficientlydynamically vary the impedance of a tank circuit so as to dynamicallyshift the f_(R) of the circuit to better match the f_(C) of a receivedsignal and thereby improve the response of the circuit.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

My invention may be more fully understood by a description of certainpreferred embodiments in conjunction with the attached drawings inwhich:

FIG. 1 is an ideal variable impedance tank circuit;

FIG. 2 is a practical embodiment of the tank circuit shown in FIG. 1;

FIG. 3 illustrates in block schematic form, a receiver circuitconstructed in accordance with a preferred embodiment of my invention;

FIG. 4 illustrates in flow diagram form the sequencing of operations inthe receiver circuit shown in FIG. 3;

FIG. 5 illustrates in block schematic form, a receiver circuitconstructed in accordance with another preferred embodiment of myinvention;

FIG. 6 illustrates in block schematic form, an alternative embodiment ofthe differentiator of FIG. 1, suitable for substitution into theembodiment shown in FIG. 5; and

FIG. 7 illustrates a preferred embodiment for the variable capacitiveand resistive elements specially adapted for use with the embodimentshown in FIG. 5.

In the drawings, similar elements will be similarly numbered wheneverpossible. However, this practice is simply for convenience of referenceand to avoid unnecessary proliferation of numbers, and is not intendedto imply or suggest that my invention requires identity in eitherfunction or structure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

As shown in the variable tank circuit 2′ in FIG. 2, in manyapplications, such as RFID tags, it may be economically desirable tosubstitute for variable inductor 4 a fixed inductor 4′. In addition, onemust take into consideration the inherent input resistance, R₁, of theload circuit 12, as well as the parasitic resistances 14 a of inductor4′ and 14 b of capacitor 6.

In accordance with the preferred embodiment of my invention as shown inFIG. 3, the amplitude modulated (“AM”) signal broadcast by the reader inan RFID system will be magnetically coupled to a conventional coilantenna comprising inductor 4′, and a portion of the induced current isextracted via nodes 8 and 10 by a regulator 16 to produce operatingpower for all other circuits. Once sufficient stable power is available,regulator 16 will produce a PowerOK signal to initiate system operation(see, 18 and 20 in FIG. 4). If desired, a variable resistor (not shown)can be provided in parallel with inductor 4′, generally between nodes 8and 10, and regulator 16 can be constructed so as to automatically varythis resistance to control the gain of the tank circuit 2′.

In response to the PowerOK signal, a timer 22 will periodically generatea timing pulse t (see, generally, 24, 26, 28, and 30 in FIG. 4).Preferably, the frequency of t pulses is a selected sub-multiple of thereceived signal, and the duty cycle is on the order of fifty percent(50%). However, as will be explained below, other duty cycles may beappropriate depending on the specific circuit elements selected toimplement my invention.

In response to the PowerOK signal, a reference voltage generator 32 willcontinuously produce a reference voltage signal V_(Ref) proportional tothe voltage induced by the received signal between nodes 8 and 10. Inresponse to the assertion of each t pulse, a differentiator 34, willsave the then-current value of the V_(Ref) signal (see, 36 in FIG. 4).Thereafter, differentiator 34 will continuously determine the polarityof the change of the previously saved value with respect to thethen-current value of the V_(Ref) signal (see, 38 in FIG. 4). If thepolarity is negative, indicating that the current V_(Ref) signal islower than the previously-saved V_(Ref) signal, differentiator 34 willassert a change direction signal; otherwise, differentiator 34 willnegate the change direction signal (see, 40 in FIG. 4).

In response to each negation of t, a direction selector 42 will togglebetween an up state and a down state if and only if differentiator 34 isthen asserting the change direction signal; otherwise, selector 42continues to maintain its current state (see, 44 in FIG. 4).

In response to the PowerOK signal, a ramp generator 46 will reset to apredetermined initial value (see, 20 in FIG. 4). Thereafter, in responseto each assertion of t, generator 46 will selectively change the valueof capacitor 6, thereby changing the resonant frequency f_(R) of circuit2′ (see, 48 in FIG. 4). Preferably, the initial value for generator 46is selected such that the initial resonant frequency f_(R) of circuit 2′will approximate the anticipated carrier frequency f_(C) of the receivedsignal, thereby assuring convergence with a minimal number of re-tuningcycles. Although the initial value can be established using any ofseveral known non-volatile techniques, including hard wiring or any of avariety of read-only-memory (ROM) structures, I prefer to use are-writable mechanism, such as a flash or otherelectrically-programmable ROM structure. Using the latter, it would be asimple matter to construct regulator 16 so as to provide a PowerLosssignal when the level of available power drops to a predeterminedminimum, and then, in response to the PowerLoss signal, to copy thecurrent value in generator 46 into the memory. Upon next receiving thePowerOK signal, the generator 46 will resume operation at the storedvalue, potentially reducing convergence time.

In accordance with my invention, after each change in the resonantfrequency f_(R) of circuit 2′, circuit 12 again determines the polarityof change of VRef. If the polarity is found to be positive, the resonantfrequency f_(R) is converging toward the carrier frequency f_(C), so thedirection of change is correct. However, if the polarity is found to benegative, the resonant frequency f_(R) is diverging from the carrierfrequency, and the direction of change must be reversed. Duringoperation, circuit 12 will selectively vary the value of capacitor 6 sothat the resonant frequency f_(R) of tank circuit 2′ converges towardthe carrier frequency f_(C) of the received signal. Thus, if thepolarity is found to be positive, circuit 12 will continue to vary thevalue of capacitor 6 in the currently-selected direction, say, forexample, “up”; but, if the polarity is found to be negative, circuit 12will switch the direction in which the value of capacitor 6 is varied,i.e., from “up” to “down”, and begin varying the value of capacitor 6 inthe newly-selected direction, now “down”. In this manner, circuit 12 isable to converge the resonant frequency f_(R) toward the carrierfrequency f_(C) regardless of whether or not the resonant frequency isinitially higher or lower than the carrier frequency.

As can be seen, I have designed circuit 12 such that it is irrelevantwhich direction is initially selected by selector 42, as circuit 12 willquickly detect divergence and reverse the state of selector 42. However,if desired, a predetermined initial direction can be selected duringinitialization using conventional means.

It is to be expected that, as difference between the resonant frequencyf_(R) of tank circuit 2′ and the carrier frequency f_(C) of the receivedsignal becomes relatively small, the ability of differentiator 34 todetect polarity changes will be significantly diminished. At such time,circuit 12 will tend to seek, i.e., changing tuning direction on each t.Additional circuitry could be easily added to detect this condition andto, for example, significantly decrease the operating frequency of timer22 or, if desired, cease operation.

Although I have heretofore described my invention in the context of ananalog embodiment, my preferred embodiment would be primarily digital.Thus, for example, in the digital circuit 12′ shown in FIG. 5, timer 22could comprise a clock 50 and an up/down-counter 52 adapted tocontinuously negate the t signal while down-counting to predeterminedminimum value and then to continuously assert the t signal whileup-counting to a predetermined maximum value, the counter 52automatically reversing count direction upon reaching the predeterminedminimum/maximum values. V_(Ref) generator 32 could be implemented usinga full-wave rectifier 54 and a low-pass filter 56, while differentiator34 could comprise a comparator 58 with its positive input adapted toreceive the current value of V_(Ref) and its negative input adapted toreceive the previous value of V_(Ref) captured and saved by asample-and-hold 60. Finally, selector 42 can be a simple toggle latch62, while generator 46 could be an n-bit, bidirectional edge-triggeredshift register 64. In response to the assertion of the PowerOK signal,shift register 64 will preferably initialize the high-order half of then-bits to logic_(—)0, and the low-order half to logic_(—)1; in responseto the leading-edge of the t signal (i.e., upon each assertion of t),shift register 64 will shift either left or right, depending on thestate of toggle latch 62. Thus, to increase frequency, register 64 wouldperform a right-shift with a left fill of logic_(—)0; whereas todecrease frequency, register 64 would perform a left-shift with aright-fill of logic_(—)1.

It will be seen that, when comparator 58 negates the change directionsignal, the resonant frequency of circuit 2″ is converging on thecarrier frequency of the received signal; whereas, when comparator 58asserts the change direction signal, the resonant frequency of circuit2″ is diverging from the carrier frequency of the received signal. Thus,for example, if the old value held in sample-and-hold 60 is less thanthe new value provided by the filter 56, comparator 58 will negate thechange direction signal, indicating that register 64 is shifting in thecorrect direction to achieve convergence; under this condition, toggle62 will not toggle. On the other hand, if the old value held insample-and-hold 60 is greater than the new value provided by the filter56, comparator 58 will assert the change direction signal, indicatingthat register 64 is not shifting in the correct direction to achieveconvergence; under this condition, toggle 62 will toggle.

In the embodiment shown in FIG. 5, I recommend selecting the minimumanticipated settling time of the sample-and-hold 60 as the minimumduration of the negated portion of each t pulse. For the period of t, Irecommend selecting the minimum anticipated settling time of the tankcircuit 2′ to each variation in tank capacitance. In such anarrangement, I would expect the negated portion of each t pulse to berelatively small with respect to the asserted portion. In general, thisarrangement should enable circuit 12′ to “re-tune” the tank circuit 2′as quickly as the various circuit components are able to detect, andthen respond to, the resulting changes in V_(Ref).

Shown in FIG. 6 is an alternative embodiment of the differentiator 34suitable for use in the circuit 12′ shown in FIG. 5. In this embodiment,I include a matched pair of sample-and-hold circuits 60 a and 60 b, eachhaving a sample input coupled to the output of the LPF 56 and a holdoutput coupled to a respective one of the +/− inputs of the comparator58. I couple the control input of sample-and-hold 60 a to the t signalsuch that, in the interval between the trailing-edge and theleading-edge of the t signal, the output of sample-and-hold 60 a willcontinuously track the “current” value of V_(Ref), whereas, in theinterval between the leading-edge and the trailing-edge of the t signal,the output of sample-and-hold 60 a will hold the value of V_(Ref) thatexisted at the leading-edge of the t signal. In contrast, I couple thecontrol input of sample-and-hold 60 b to the inverse of t signal sothat, in the interval between the leading-edge and the trailing-edge ofthe t signal, the output of sample-and-hold 60 b will continuously trackthe “current” value of V_(Ref), whereas, in the interval between thetrailing-edge and the leading-edge of the t signal, the output ofsample-and-hold 60 b will hold the value of V_(Ref) that existed at thetrailing-edge of the t signal. Thus, during each half-cycle of t,comparator 58 will be comparing the “new” value of V_(Ref) to the “old”value saved in a respective one of the sample-and-holds 60 a and 60 b.To account for the fixed +/− polarity of the inputs of comparator 58, Ihave coupled the output of comparator 58 to a first input of anexclusive-or circuit, XOR 66, and to the other input of XOR 66 I havecoupled t. In operation, the output of XOR 66 will be asserted only ifthe new value of V_(Ref) is determined by comparator 58 to be less thanthe old value of V_(Ref). Since this embodiment (and itslogical/functional equivalents) is capable of performing a newcomparison on each edge of t, both toggle 62 and register 64 must bemodified so as also to be responsive to both edges of t. In thisembodiment, I prefer the duty-cycle of t to be approximately fiftypercent (50%).

Rather than use the XOR 66, it would be possible to provide amultiplexor (not shown), responsive to the t signal, to alternatelycouple then cross-couple the hold outputs of the sample-and-holds 60 aand 60 b such that the old value of V_(Ref) is always coupled to thenegative input of comparator 58 and the new value of V_(Ref) is alwayscoupled to the positive input of comparator 58. In such an arrangement,the polarity of the output of comparator 58 will always be consistentwithout regard to which sample-and-hold is holding the old value ofV_(Ref); that is, the output of comparator 58 will always be positive ifthe new value of V_(Ref) is greater than the old value, and negative ifthe new value is less than the old value. Alternatively, it would alsobe possible to provide a multiplexor (not shown), responsive to the tsignal, to alternately couple the output of comparator 58 and theinverse thereof to the input of toggle 62. In both of these alternateembodiments (and their logical/functional equivalents), I would againrecommend the duty-cycle of t to be approximately fifty percent (50%).

In accordance with a preferred embodiment of my invention as shown inFIG. 7, each of the n bits in register 64 controls, at a minimum, arespective one of n solid-state switches 68 a through 68 n. Thus, forexample, in my tank circuit 2″, I start with a fixed capacitor 70selected to provide what I expect to be the minimum required capacitance(i.e., maximum resonant frequency) for anticipated operating conditions.I then provide n smaller capacitors 72 a through 72 n, the sum of whosecapacitances, when added to the capacitance of capacitor 70, representwhat I expect to be the maximum required capacitance (i.e., minimumresonant frequency) for anticipated operating conditions. During systemoperation, each of the n switches 68 a through 68 n is opened/closed byan output from a respective one of the n bits in register 64, therebyselectively adding the respective capacitor 72 a-72 n to the circuit 2″.

As can be seen from Eq. 1 and Eq. 2, adding capacitance to circuit 2″decreases the resonant frequency, while subtracting capacitanceincreases the resonant frequency. Remember that I have initializedregister 64 such that a predetermined number of all of the switchedcapacitors 72 a-72 n are initially switched into circuit 2″. Ifcapacitors 70 and 72 a-72 n has been properly selected, the initialresonant frequency of circuit 2″ will be approximately equal to theexpected carrier frequency. Thus, convergence can be efficientlyachieved by gradually adding/subtracting capacitors 72 a through 72 nuntil, at nearest convergence, toggle 62 will begin dithering. As notedabove, although not essential, additional circuitry could be added todetect this condition and significantly drop the rate of sampling, forexample, by dynamically changing the minimum/maximum count values ofcounter 52. It would also be possible to terminate tuning altogether,but at the risk of losing sync due to unexpected changes in the systemoperating characteristics, such as might result from physical movementof the tag to another location within the perimeter of the system.

Thus it is apparent that I have provided an efficient method andapparatus for dynamically varying the impedance of a tank circuit, and,in particular, wherein the impedance of the circuit can be efficientlyvaried so as to dynamically shift the f_(R) of the circuit to bettermatch the f_(C) of a received signal and thereby improve the response ofthe circuit. Those skilled in the art will recognize that modificationsand variations can be made without departing from the spirit of myinvention. For example, although in the embodiment I have shown in FIG.5 I chose to implement the non-linear transfer function performed by myreference voltage generator 32 using a combination of full-waverectifier 54 and low-pass filter 56, there exist many effectivesubstitutions, including, for example, a multiplier, such as thewell-known Gilbert Multiplier Circuit (shown and described in Section2.5 of “An Analog Cell Library Useful for Artificial Neural Networks”,IEEE Proceedings—1990 Southeastcon, a copy of which is submittedherewith and incorporated herein in its entirety by reference).Similarly, I recognize that additional signal conditioning circuits,gain stages, and the like, may be added, if desired, in the designs ofpractical, robust, commercial implementations. Further, although Iprefer to measure the response of the tank circuit 2′ using a voltagereference generator 32, it would be possible to develop a suitablereference that is proportional to the phase difference between theresonant frequency f_(R) and the carrier frequency f_(C). Therefore, Iintend that my invention encompass all such variations and modificationsas fall within the scope of the appended claims.

1. A tuning circuit for dynamically varying the impedance of a tankcircuit comprising an inductor and a capacitor, the capacitance of whichcan be varied in a selected one of first and second directions, thetuning circuit comprising: a reference voltage generator, coupled to thetank circuit, to produce a reference voltage proportional to theresponse of the tank circuit to a received signal, wherein the voltagegenerator comprises: a rectifier, coupled to the tank circuit; and a lowpass filter, coupled to the rectifier; a differentiator, coupled to thegenerator, to determine a polarity of the change in the referencevoltage between a first point in time and a second point in time,wherein the differentiator comprises: a sample-and-hold, coupled to thevoltage generator, to sample the reference voltage at said first pointin time; and a comparator, having a first input coupled to thesample-and-hold, and a second input coupled to the voltage generator; adirection selector, coupled to the differentiator, to select one of saidfirst and second directions in response to said polarity, wherein thedirection selector comprises: a toggle, coupled to the differentiator,to toggle between said first and second directions in response to anegative polarity; and a ramp generator, coupled to the selector and tothe capacitor, to selectively vary the capacitance of said capacitor insaid selected direction, wherein the ramp generator comprises: a shiftregister, coupled to the direction selector.
 2. A method for dynamicallyvarying the impedance of a tank circuit comprising an inductor and acapacitor, the capacitance of which can be varied in a selected one offirst and second directions, the method comprising the steps of: (1)selecting, at random, one of said first and second directions; (2)capturing a current response of the circuit to a received signal; (3)varying the capacitor in said selected direction; (4) comparing thecaptured response to the current response; (5) if the comparison made instep 4 indicates that the current response is weaker than the capturedresponse, selecting the other of said directions; and (6) returning tostep
 2. 3. The method of claim 2 wherein step 6 is further characterizedas: (6) if the comparison made in step 4 indicates that the differencebetween the current response and the captured response is greater than apredetermined minimum, returning to step
 2. 4. The method of claim 2wherein step 6 is further characterized as: (6) if the comparison madein step 4 indicates that the difference between the current response andthe captured response is greater than a predetermined minimum,selectively returning to step
 2. 5. A tuning circuit for dynamicallyvarying the impedance of a tank circuit comprising an inductor and acapacitor, the capacitance of which can be varied in a selected one offirst and second directions, the tuning circuit comprising: a referencevoltage generator, coupled to the tank circuit, to produce a referencevoltage proportional to the response of the tank circuit to a receivedsignal; a differentiator, coupled to the generator, to determine apolarity of the change in the reference voltage between a first point intime and a second point in time; a direction selector, coupled to thedifferentiator, adapted to select, at random, one of said first andsecond directions in response to said polarity prior to said secondpoint in time; and a ramp generator, coupled to the selector and to thecapacitor, to selectively vary the capacitance of said capacitor in saidselected direction.
 6. The circuit of claim 5 wherein the directionselector is adapted to select the other of said first and seconddirections in response to a negative polarity.
 7. The circuit of claim 5wherein the voltage generator comprises: a rectifier, coupled to thetank circuit; and a low pass filter, coupled to the rectifier.
 8. Thecircuit of claim 5 wherein the differentiator comprises: asample-and-hold, coupled to the voltage generator, to sample thereference voltage at said first point in time; and a comparator, havinga first input coupled to the sample-and-hold, and a second input coupledto the voltage generator.
 9. The circuit of claim 5 wherein thedirection selector comprises: a toggle, coupled to the differentiator,to toggle between said first and second directions in response to anegative polarity.
 10. The circuit of claim 5 wherein the ramp generatorcomprises: a shift register, coupled to the direction selector.
 11. Thecircuit of claim 5 wherein the voltage generator comprises: a non-lineartransfer circuit, coupled to the tank circuit.
 12. A method fordynamically varying the impedance of a tank circuit comprising aninductor and a capacitor, the capacitance of which can be varied in aselected one of first and second directions, the method comprising thesteps of: (1) selecting, at random, one of said first and seconddirections; (2) capturing a current response of the circuit to a signalreceived via said indicator; (3) varying the capacitor in said selecteddirection; (4) comparing the captured response to the current response;(5) if the comparison made in step 4 indicates that the current responseis weaker than the captured response, selecting the other of saiddirections; and (6) returning to step
 2. 13. The method of claim 12wherein step 6 is further characterized as: (6) if the comparison madein step 4 indicates that the difference between the current response andthe captured response is greater than a predetermined minimum, returningto step
 2. 14. The method of claim 12 wherein step 6 is furthercharacterized as: (6) if the comparison made in step 4 indicates thatthe difference between the current response and the captured response isgreater than a predetermined minimum, selectively returning to step 2.15. A tuning circuit for dynamically varying the impedance of a tankcircuit comprising an inductor and a capacitor, the capacitance of whichcan be varied in a selected one of first and second directions, thetuning circuit comprising: a reference voltage generator, coupled to thetank circuit, to produce a reference voltage proportional to theresponse of the tank circuit to a signal received via said inductor; adifferentiator, coupled to the generator, to determine a polarity of thechange in the reference voltage between a first point in time and asecond point in time; a direction selector, coupled to thedifferentiator, adapted to select, at random, one of said first andsecond directions in response to said polarity prior to said secondpoint in time; and a ramp generator, coupled to the selector and to thecapacitor, to selectively vary the capacitance of said capacitor in saidselected direction.
 16. The circuit of claim 15 wherein the directionselector is adapted to select the other of said first and seconddirections in response to a negative polarity.
 17. The circuit of claim15 wherein the voltage generator comprises: a rectifier, coupled to thetank circuit; and a low pass filter, coupled to the rectifier.
 18. Thecircuit of claim 15 wherein the differentiator comprises: asample-and-hold, coupled to the voltage generator, to sample thereference voltage at said first point in time; and a comparator, havinga first input coupled to the sample-and-hold, and a second input coupledto the voltage generator.
 19. The circuit of claim 15 wherein thedirection selector comprises: a toggle, coupled to the differentiator,to toggle between said first and second directions in response to anegative polarity.
 20. The circuit of claim 15 wherein the rampgenerator comprises: a shift register, coupled to the directionselector.
 21. The circuit of claim 15 wherein the voltage generatorcomprises: a non-linear transfer circuit, coupled to the tank circuit.22. A method for dynamically varying the impedance of a tank circuitcomprising an inductor and a capacitor coupled in parallel with saidinductor, the capacitance of which can be varied in a selected one offirst and second directions, the method comprising the steps of: (1)selecting, at random, one of said first and second directions; (2)capturing a current response of the circuit to a received signal; (3)varying the capacitor in said selected direction; (4) comparing thecaptured response to the current response; (5) if the comparison made instep 4 indicates that the current response is weaker than the capturedresponse, selecting the other of said directions; and (6) returning tostep
 2. 23. The method of claim 22 wherein step 6 is furthercharacterized as: (6) if the comparison made in step 4 indicates thatthe difference between the current response and the captured response isgreater than a predetermined minimum, returning to step
 2. 24. Themethod of claim 22 wherein step 6 is further characterized as: (6) ifthe comparison made in step 4 indicates that the difference between thecurrent response and the captured response is greater than apredetermined minimum, selectively returning to step
 2. 25. A tuningcircuit for dynamically varying the impedance of a tank circuitcomprising an inductor and a capacitor coupled in parallel with saidinductor, the capacitance of which can be varied in a selected one offirst and second directions, the tuning circuit comprising: a referencevoltage generator, coupled to the tank circuit, to produce a referencevoltage proportional to the response of the tank circuit to a receivedsignal; a differentiator, coupled to the generator, to determine apolarity of the change in the reference voltage between a first point intime and a second point in time; a direction selector, coupled to thedifferentiator, adapted to select, at random, one of said first andsecond directions in response to said polarity prior to said secondpoint in and a ramp generator, coupled to the selector and to thecapacitor, to selectively vary the capacitance of said capacitor in saidselected direction.
 26. The circuit of claim 25 wherein the directionselector is adapted to select the other of said first and seconddirections in response to a negative polarity.
 27. The circuit of claim25 wherein the voltage generator comprises: a rectifier, coupled to thetank circuit; and a low pass filter, coupled to the rectifier.
 28. Thecircuit of claim 25 wherein the differentiator comprises: asample-and-hold, coupled to the voltage generator, to sample thereference voltage at said first point in time; and a comparator, havinga first input coupled to the sample-and-hold, and a second input coupledto the voltage generator.
 29. The circuit of claim 25 wherein thedirection selector comprises: a toggle, coupled to the differentiator,to toggle between said first and second directions in response to anegative polarity.
 30. The circuit of claim 25 wherein the rampgenerator comprises: a shift register, coupled to the directionselector.
 31. The circuit of claim 25 wherein the voltage generatorcomprises: a non-linear transfer circuit, coupled to the tank circuit.